Consider pachymeter types, corneal factors when measuring corneal thickness
Elliot M. Kirstein
Measuring corneal thickness involves not only choosing from several pachymeter options, but also considering how contact lens wear or the presence of ocular disorders may affect the reading.
Ultrasound pachymetry is popular due to its ease of use, affordability and repeatability, Elliot M. Kirstein, OD, FAAO, director at Harpers Point Eye Associates in Cincinnati, told Primary Care Optometry News. I would imagine ultrasound represents about 98% of the market.
Murray Fingeret, OD, chief of optometry at the Department of Veterans Administration New York Harbor Healthcare System in Brooklyn, agrees. Ultra-sound is really the only technology being used today because it is easier to use and more accurate, he said in an interview.
Dr. Fingeret said optical pachymetry is an old technology. Im not aware of anyone who still uses it, he said.
According to J. James Thimons, OD, medical director at Ophthalmic Consultants of Connecticut, in Fairfield, pachymeters such as scanning-slit types use optical principals to deduce relative differences in corneal thickness. However, I dont think these types of pachymeters are as accurate as some of the other technologies.
J. James Thimons
For instance, the ultrasonic probes provide the best density analysis of the tissue, Dr. Thimons said in an interview. They can analyze both front and back surface reflectance properties accurately. From my perspective, ultrasonic pachymeters are the most accurate of the pachymeters available.
Among the varied technologies, optical pachymeters tend to read thinner in my experience, Dr. Thimons said. Papers published, including one of mine presented at the American Academy of Ophthalmology meeting, showed that the average cornea was about 20 microns thinner.
A nomogram can compensate for this discrepancy, he said.
Overall, Dr. Thimons prefers the ultrasound systems because they correlate with the Ocular Hypertension Treatment Study, which is the baseline for all the basic decisions about treating glaucoma patients, he said.
Randall K. Thomas, OD, who practices in Concord, N.C., said that the type of pachymeter used is clinically irrelevant. A difference of 2 or 3 or 5 or even 10 mm is rarely important, he said.
Dr. Fingeret said that newer pachymeters take multiple measurements at one time. As long as the standard deviation is relatively low preferably under 5 microns these measurements are accurate, he said.
New imaging devices using Scheimpflug cameras show corneal thickness from limbus to limbus, Dr. Kirstein added. These devices take much more reliable measurements than handheld pachymeters, he said.
Thick corneas, thin corneas
A number of factors can cause a cornea to be thicker or thinner than average, according to Dr. Kirstein, research coordinator at Ziemer Ophthalmology. Certainly, the cornea varies in thickness with time of day or from an ongoing disease process, he said. Contact lens wear can also cause the cornea to nominally change in thickness.
Heredity and ocular medications may be factors as well. Refractive surgery is definitely a factor, he said.
Anything that can affect the cornea will influence corneal thickness and influence the measurement, Dr. Fingeret added. For instance, any corneal infections may affect thickness, he said.
Dr. Thomas believes that, in most cases, contact lenses or early-morning corneal edema have no clinical bearing on pachymetry measurements. We are only interested in whether the cornea is thin, normal or thick, he told PCON. Thin corneas carry an increased risk for glaucoma. We also use corneal thickness readings to help us obtain a more refined reading of the intraocular pressure. A thick cornea tends to make the pressures seem artificially high, whereas thinner corneas underestimate the IOP.
Patients with atypical corneas, such as keratoconus or forme fruste corneal disease, can have thin corneas because of their corneal diseases that are unrelated to glaucoma.
Individuals with a history of Fuchs corneal dystrophy or anterior basement membrane dystrophy also have to be looked at relative to clinical interpretation, Dr. Thimons said. For Fuchs, I would probably automatically discard the measurement, because the disease causes corneal swelling that eliminates the usefulness of the data to predict potential glaucoma status.
Timing of measurement
There is no preferred time of day to perform pachymetry, according to Dr. Kirstein. However, timing is crucial when determining how much swelling contact lens wear causes. The eye should be measured right after the lens is removed, then perhaps 1 or 2 hours later when the cornea has recovered to its original thickness, he said.
Susan A. Resnick
Similarly, during sleep, the cornea swells slightly. The recommendation is to wait 2 hours after a person is awake to perform pachymetry, Dr. Fingeret said. Other than that, there is no one specific time.
He agreed with Dr. Kirstein in saying that contact lenses wearers should remove their lenses 2 hours before they are measured.
Dr. Thomas believes time of day and contact lens wear are relatively insignificant factors. I do not believe lens wearers should remove their lenses for a certain period of time before measuring, he said.
A condition such as Fuchs endothelial dystrophy will likely result in a higher reading in the morning, according to Susan A. Resnick, OD, FAAO, a New York private practitioner. In the context of everyday practice, however, I dont think most practitioners care about the time, she told PCON.
Guidelines for lens wearers
According to Dr. Resnick, You might find a thinner pachymetry [in GP lens wearers] than in those who do not wear lenses, she said. Temporary epithelial displacement may occur under the lens, which can obviously influence the thickness reading in the central part of the cornea.
Conversely, soft lenses associated with any clinically significant edema results in a higher pachymetry measurement, said Dr. Resnick, a contact lens specialist. However, todays advanced contact lens technology makes lens wear much less of a factor. It would be rare to find a pachymetry significantly influenced by soft lenses, especially the silicone hydrogels, she said.
When performing pachymetry as part of assessing a candidates suitability for refractive surgery, Dr. Resnick recommends that the patient remove his or her soft lenses 3 days prior to measuring; for hard lenses the period is longer. She also schedules pachymetry before fitting patients with overnight orthokeratology. The central cornea thins a bit throughout the process, she said. We often find that patients who start overnight orthokeratology with very thin corneas sometimes dont end up with as effective of a result.
Dr. Kirstein commented on reimbursement for patients with ocular hypertension. My understanding is that pachymetry is reimbursed once per lifetime per practice, he said. We have no obligation to call another practice to find out that measurement or to assume that it was billed for or not.
If the insurance company denies a clinician reimbursement, the doctor can appeal, Dr. Fingeret said.
Pachymetry: Easy, necessary
Pachymetry has now become part of an eye examination for anyone with glaucoma or ocular hypertension, said Dr. Fingeret. We know that corneal thickness influences IOP measurements, as well as being an independent risk factor for the development of glaucoma, he said.
Dr. Thomas stated that both optometrists and ophthalmologists underuse pachymetry. To maximize patient care, all eye doctors need to have pachymetry technology readily available and use it in the diagnostic work-up of all glaucoma suspects. It is an extremely easy procedure, he said. Any high school graduate can be trained to do it well in about an hour.
For more information:
- Elliot M. Kirstein, OD, FAAO, can be reached at 8211 Cornell Rd., Cincinnati, Ohio 45249-2313; (513) 530-0440; fax: (513) 530-0473; e-mail: email@example.com.
- Murray Fingeret, OD, is a PCON Editorial Board member. He may be contacted at St. Albans VA Hospital, Linden Blvd. & 179th St., St. Albans, NY 11425; (718) 298-8498; fax: (516) 569-3566; e-mail: firstname.lastname@example.org.
- J. James Thimons, OD, is a PCON Editorial Board member. He can be reached at Ophthalmic Consultants of Connecticut, 75 Kings Highway Cutoff, Fairfield, CT 06430; (203) 257-7336; fax: (203) 330-4958; e-mail: email@example.com.
- Randall K. Thomas, OD, is a PCON Editorial Board member. He can be reached at 6017 Havencrest Ct., Concord, NC 28027; (704) 792-6021; fax: (704) 792-1647; e-mail: firstname.lastname@example.org.
- Susan A. Resnick, OD, FAAO, can be reached at 30 E. 60th St., New York, NY 10022; (212) 355-5145; fax: (212) 308-3262; e-mail: email@example.com.